Imho a screenshot of taskmanager will help the devs to help you to find the solution.
I guess you got out of RAM, it makes the cpu run in low percentage and low at render passes/rays and excessive parsing time.
....it seems Corona 1.5 is not capable of rendering large images...
Wihout out of ram, afaik a lot of people here done their huge renders savely, huge means larger than 4000px to 30000px. My largest render was 8000 x 8000px :)
But there was an issue about a type of hi end cpu which is weirdly run in low percentage because it has too much cores, i'm not sure how to describe this kind of problem but you can mention your cpu type too